Prefetch management in a hierarchical cache system

ABSTRACT

An apparatus includes a CPU core, a first memory cache with a first line size, and a second memory cache having a second line size larger than the first line size. Each line of the second memory cache includes an upper half and a lower half. A memory controller subsystem is coupled to the CPU core and to the first and second memory caches. Upon a miss in the first memory cache for a first target address, the memory controller subsystem determines that the first target address resulting in the miss maps to the lower half of a line in the second memory cache, retrieves the entire line from the second memory cache, and returns the entire line from the second memory cache to the first memory cache.

CROSS REFERENCE TO RELATED APPLICATIONS

This disclosure contains subject matter that may be related to the subject matter of U.S. patent application Ser. No. 16,102,931, filed Aug. 14, 2018, entitled “Prefetch Kill and Revival in an Instruction Cache”.

BACKGROUND

Some memory systems include a multi-level cache system. Upon receipt from a processor core by a memory controller of a request for a particular memory address, the memory controller determines if data associated with the memory address is present in a first level cache (L1), If the data is present in the L1 cache, the data is returned from the LI cache. If the data associated with the memory address is not present in the L1 cache, then the memory controller accesses a second level cache (L2) which may be larger and thus hold more data than the LI cache. If the data is present in the L2 cache, the data is returned from the L2 cache to the processor core and a copy also is stored in the L1 cache in the event that the same data is again requested. Additional memory levels of the hierarchy are possible as well.

SUMMARY

In at least one example, an apparatus includes a central processing unit (CPU) core, a first memory cache to store instructions for execution by the CPU core, the first memory cache configured to have a first line size, and a second memory cache to store instructions for execution by the CPU core. The second memory cache has a second line size that is larger than the first line size, and each line of the second memory cache comprising an upper half and a lower half. A memory controller subsystem is coupled to the CPU core and to the first and second memory caches. Upon a miss in the first memory cache for a first target address, the memory controller subsystem determines that the first target address resulting in the miss maps to the lower half of a line in the second memory cache, retrieves the entire line from the second memory cache, and returns the entire line from the second memory cache to the first memory cache.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now be made to the accompanying drawings in which:

FIG. 1 illustrates a processor in accordance with an example.

FIG. 2 illustrates the promotion of an L1 memory cache access to a full L2 cache line access in accordance with an example.

FIG. 3 is a flow chart to illustrate a performance improvement in accordance with an example.

FIG. 4 is another flow chart to illustrate another performance improvement in accordance with an example.

FIG. 5 shows a system that includes the processor of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an example of a processor 100 that includes a hierarchical cache subsystem. The processor 100 in this example includes a central processing unit (CPU) core 102, a memory controller subsystem 101, an L1 data cache (L1D) 115, an L1 program cache (L1P) 130, and an L2 memory cache 155. In this example, the memory controller subsystem 101 includes a data memory controller (DMC) 110, a program memory controller (PMC) 120, and a unified memory controller (UMC) 150. In this example, at the L1 cache level, data and program instructions are divided into separate caches. Instructions to be executed by the CPU core 102 are stored in the L1P 130 to then be provided to the CPU core 102 for execution. Data, on the other hand, is stored in the L1D 115. The CPU core 102 can read data from, or write data to, the L1D 115 but has read access to (no write access to) the L1P 130. The L2 memory cache 155 can store both data and program instructions.

Although the sizes of the L1D 115, L1P 130, and L2 memory cache 155 can vary from implementation to implementation, in one example, the size of the L2 memory cache 155 is larger than the size of either the L1D 115 or the L1P 130. For example, the size of the L1D 115 is 32 kbytes and the size of the L1P also is 32 kbytes, while the size of the L2 memory cache can between 64 kbytes and 4 MB. Further, the cache line size of the L1D 115 is the same as the cache line size of the L2 memory cache 155 (e.g., 128 bytes), and the cache line size of the L1P 130 is smaller (e.g., 64 bytes).

Upon the need by the CPU core 102 for data, the DMC 110 receives an access request for the target data from the CPU core 102. The access request may comprise an address (e.g., a virtual address) from the CPU core 102. The DMC 110 determines whether the target data is present in the L1D 115. If the data is present in the L1D 115, the data is returned to the CPU core 102. If, however, the data requested by the CPU core 102 is not present in the L1D 115, the DMC 110 provides an access request to the UMC 150. The access request may comprise a physical address that is generated by the DMC 110 based on the virtual address (VA) provided by the CPU core 102. The UMC 150 determines whether the physical address provided by the DMC 110 is present in the L2 memory cache 155. If the data is present in the L2 memory cache 155, the data is returned to the CPU core 102 from the L2 memory cache 155 with a copy being stored in the L1D 115. An additional hierarchy of the cache subsystem may be present as well. For example, an L3 memory cache or system memory may be available to be accessed. As such, if the data requested by the CPU core 102 is not present in either the L1D 115 or the L2 memory cache 155, the data can be accessed in an additional cache level.

With regard to program instructions, when the CPU core 102 needs additional instructions to execute, the CPU core 102 provides a VA 103 to the PMC 120. The PMC responds to the VA 103 provided by the CPU core 102 by initiating a work flow to return a prefetch packet 105 of program instructions back to the CPU core 102 for execution. Although the size of the prefetch packet can vary from implementation to implementation, in one example, the size of the prefetch packet equals the size of the cache line of the L1P 130. If the L1P cache line size is, for example, 64 bytes, a prefetch packet returned to the CPU core 102 will also contain 64 bytes of program instructions.

The CPU core 102 also provides a prefetch count 104 to the PMC 120. In some implementations, the prefetch count 104 is provided to the PMC 120 after the CPU core 102 provides the VA 103. The prefetch count 104 indicates the number of prefetch units of program instructions following the prefetch unit starting at the VA 103. For example, the CPU core 102 may provide a VA of 200 h. That VA is associated with a prefetch unit of 64 bytes that begins at virtual address 200 h. If the CPU core 102 wants the memory controller subsystem 101 to send additional instructions for execution following the prefetch unit associated with virtual address 200 h, the CPU core 102 submits a prefetch count with a value greater than 0. A prefetch count of 0 means that the CPU core 102 does not need any more prefetch units. A prefetch count of, for example, 6 means that the CPU core 102 requests an additional 6 prefetch units worth of instructions to be obtained and sent back to the CPU core 102 for execution. The return prefetch units are shown in FIG. 1 as prefetch packets 105.

Referring still to the example of FIG. 1, the PMC 120 includes a TAGRAM 121, an address translator 122, and a register 123. The TAGRAM 121 includes a list of the virtual addresses whose contents (program instructions) have been cached to the L1P 130. The address translator 122 translates virtual addresses to physical addresses (PAs). In one example, the address translator 122 generates the physical address directly from the virtual address. For example, the lower 12 bits of the VA may be used as the least significant 12 bits of the PA, with the most significant bits of the PA (above the lower 12 bits) being generated based on a set of tables configured in main memory prior to execution of the program. In this example, the L2 memory cache 155 is addressable using physical addresses, not virtual addresses. The register 123 stores a hit/miss indicator 124 from a TAGRAM 121 look-up, the physical address 125 generated by the address translator 122 and a valid bit 126 (also referred to herein as a status bit) to indicate whether the corresponding hit/miss indicator 124 and physical address 125 are valid or invalid.

Upon receipt of a VA 103 from the CPU 102, the PMC 120 performs a TAGRAM 121 look-up to determine whether the L1P 130 includes program instructions associated with that virtual address. The result of the TAGRAM look-up is a hit or miss indicator 124. A hit means that the VA is present in the L1P 130 and a miss means that the VA is not present in the L1P 130. For an L1P 130 hit, the target prefetch unit is retrieved by the PMC 120 from the L1P 130 and returned as a prefetch packet 105 to the CPU core 102.

For an L1P 130 miss, the PA (generated based on the VA) is provided by the PMC 120 to the UMC 150 as shown at 142. A byte count 140 also is provided from the PMC 120 to the UMC 150. The byte count indicates the number of bytes of the L2 memory cache 155 that is to be retrieved (if present) starting at the PA 142. In one example, the byte count 140 is a multi-bit signal that encodes the number of bytes desired from the L2 memory cache 155. In an example, the line size of the L2 memory cache is 128 bytes, and each line is divided into an upper half (64 bytes) and a lower half (64 bytes). The byte count 140 thus may encode the number 64 (if only the upper or lower half 64 bytes are needed from a given L2 memory cache line) or 128 (if an entire L2 memory cache line is needed). In another example, the byte count may be a single bit signal where one state (e.g., 1) implicitly encodes an entire L2 memory cache line and another state (e.g., 0) implicitly encodes half of an L2 memory cache line.

The UMC 150 also includes a TAGRAM 152. The PA 142 received by the UMC 150 from the PMC 120 is used to perform a look-up into the TAGRAM 152 to determine whether the target PA is a hit or a miss in the L2 memory cache 155. If there is a hit in the L2 memory cache 155, the target information, which may be one-half of the cache line or the entire cache line depending on the byte count 140, the target information is returned to the CPU core 102 with a copy being stored in the L1P 130 from which the same program instructions will be provided to the CPU core 102 the next time that the CPU core 102 attempts to fetch the same program instructions.

In example of FIG. 1, the CPU core 102 provides a VA 103 and a prefetch count 104 to the PMC 120. The PMC 120 initiates the workflow to retrieve the prefetch packet from the L1P 130 or L2 memory cache 155 as describe above. Using the prefetch count 104 and the original VA 103, the PMC 120 calculates additional virtual addresses and proceeds to retrieve prefetch packets corresponding to those calculated VAs from the L1P 130 or L2 memory cache 155. For example, if the prefetch count is 2 and the VA 103 from the CPU core 102 is 200 h, the PMC 120 calculates the next two VAs as 240 h and 280 h, rather than the CPU core 102 providing each such VA to the PMC 120.

FIG. 2 illustrates a specific example in which an optimization results in improved performance of the processor 100. As noted above, the line width of L2 memory cache 155 is larger than the line width of L1P. In one example, the width of L1P is 64 bytes and the line width of L2 memory cache 155 is 128 bytes as shown in FIG. 2. The L2 memory cache 155 is organized as an upper half 220 and a lower half 225. The UMC 150 can read an entire 128 byte cache line from the L2 memory cache 155, or only half (upper half 220 or lower half 225) of an L2 memory cache line.

A given VA may translate into a particular PA that, if present in the L2 memory cache 155, maps to the lower half 225 of a given line of the L2 memory cache or maps to the upper half 220. Based on the addressing scheme used to represent VAs and PAs, the PMC 120 can determine whether a given VA would map to the lower half 225 or upper half 220. For example, a particular bit within the VA (e.g., bit 6) can be used to determine whether the corresponding PA would map to the upper or lower halves of the line of the L2 memory cache. For example, bit 6 being a 0 may indicate the lower half and bit 6 being a 1 may indicate the upper half.

Reference numeral 202 shows an example of a VA of 200 h provided by the CPU core 102 to the PMC 120 and a corresponding prefetch count of 6. Reference numeral 210 illustrates that the list of VAs that are run through the cache pipeline described above include 200 h (received from the CPU core 102) and the next 6 consecutive virtual address 240 h, 280 h, 2 c 0 h, 300 h, 340 h, and 380 h (calculated by the PMC 120).

Each address from 200 h through 380 h is processed as described above. Any or all of the VAs may be a miss in the L1P 130. The PMC 120 can package two consecutive VAs that miss in the L1P 130 into a single L2 cache line access attempt. That is, if 200 h and 240 h both miss in the L1P 130, and the physical address corresponding to 200 h corresponds to the lower half 225 of a particular cache line of the L2 memory cache 155 and the physical address corresponding to 240 h corresponds to the upper half 225 of the same cache line of the L2 memory cache, the PMC 120 can issue a single PA 142 to the UMC 150 along with a byte count 140 specifying an entire cache line from the L2 memory cache. That is, two contiguous VA misses in L1P 130 can be promoted into a single full line L2 memory cache look-up.

If the last VA in a series of VAs initiated by the CPU core 102 (e.g., VA 380 h in VA series 210) maps to the lower half 225 of a cache line of the L2 memory cache 155, then in accordance with the described examples, the entire cache line of the L2 memory cache 155 is retrieved even though only the lower half 225 was needed. The same response occurs if the CPU provided a VA 103 to the PMC 120 with prefetch count of 0 meaning that the CPU 102 only wanted a single prefetch unit. Very little if any additional overhead, time, or power consumption is expended to retrieve the entire cache line and provide the entire cache line to the L1P 130. Since program instructions are often executed in linear order, the probability is generally high that the program instructions in the upper half 220 would be executed following the execution of the instructions in the lower half 225 anyway. Thus, the next set of instructions are received at very little cost and such instructions are likely to be needed anyway.

FIG. 2 illustrates through arrow 213 that VA 380 h maps to the lower half 225 of cache line 260 in the L2 memory cache 155. The PMC 120 determines this mapping through an examination, for example, of one or more of the bits of the VA or its counterpart physical address following translation by the address translator 122. The PMC 120 promotes the look-up process by the UMC 150 to a full cache line read by submitting the PA associated with VA 380 h along with a byte count 104 that specifies the entire cache line. The entire 128 byte cache line (if present in the L2 memory cache 155) is then retrieved and written to the L1P 130 in two separate 64 byte cache lines as indicated at 265.

If, however, the last VA in the series of VAs (or if there is only one VA for a prefetch count of 0) maps to the upper half 220 of a cache line of the L2 memory cache 155, then the PMC 120 requests the UMC 150 to look-up in its TAGRAM 152 and return to the CPU core 102 and the L1P 130 only the upper half of the cache line. The next PA would be in the lower half 225 of the next cache line of the L2 memory cache 155 and additional time, overhead and power would be consumed to speculatively retrieve the next cache line, and it is not certain that the CPU core 102 would need to execute those instructions.

FIG. 3 shows an example of a flow chart 300 for the above-described method. The operations can be performed in the order shown, or in a different order. Further, the operations can be performed sequentially or two or more of the operations can be performed concurrently.

At 302, the method includes receiving by the memory controller subsystem 101 an access request for N prefetch units of program instructions. In one implementation, this operation is performed by the CPU core 102 providing an address and count value to the PMC 120. The address may be a virtual address or a physical address, and the count value may indicate the number of additional prefetch units that are needed by the CPU core 102.

At 304, an index value I is initialized to a value 1. This index value is used to determine when the last virtual address in a series of consecutive virtual addresses is to be processed by the PMC 120. At 306, the method determines whether prefetch unit I is a hit or a miss into the L1P 130. This determination is made in some examples by determining if the virtual address is present in the PMC's TAGRAM 121. Two results are possible from the determination 306—a hit or a miss.

If the virtual address is a hit into the L1P 130, then at 308, the corresponding line of the L1P 130 which contains the desired prefetch unit is returned from the L1P 130 and provided back to the CPU core 102 as a prefetch packet 105. The index is then incremented at 310 (I=I+1). If I has not yet reached N+1 (as determined at decision operation 312), then the VA of the last of the prefetch units has not yet been evaluated for the hit/miss determination, and control loops back to 306 to evaluate the next Ith prefetch unit for a hit or miss in the L1P 130. If I has reached N+1, then all N prefetch units have been evaluated and the corresponding program instructions have been supplied to the CPU core 102 and the process stops.

For a given Ith prefetch unit, if PMC 120 determines there to be a miss in the L1P 130 at 306, then a determination is made at 314 as to whether I has reached the value of N. If I does not equal N (indicating that the last VA in a series of VAs has not been reached), then at 316, the method includes the memory controller subsystem 101 obtaining program instructions from the L2 memory cache 155 (if present there, or from a third level cache or system memory if not present). The index value I is then incremented at 318 and control loops back to determination 306.

If, at 314, I has reached N (indicating that the last VA in the series of VAs has been reached), then at 320 the method includes determining whether the VA of the Ith prefetch unit maps to the lower half or the upper half of the cache line of the L2 memory cache 155. An example of how this determination can be made is described above. If the VA of the Ith prefetch unit maps to the upper half, then at 322 the method includes obtaining the program instructions from only the upper half of the cache line of the L2 memory cache.

However, if the VA of Ith prefetch unit maps to the lower half, then at 324 the method includes promoting the L2 memory cache access to a full cache line access and, at 326, obtaining the program instructions from the full cache line of the L2 memory cache.

Referring back to FIG. 1, as described above, following submission from the CPU core 102 to the PMC 120 of a VA 103, the CPU core 102 also can provide a prefetch count 104 to the PMC 120. The prefetch count could be 0 meaning that the CPU core 102 does not need any more instructions other than those contained in the prefetch unit starting at the VA 103. However, in between receipt of the VA 103 and the subsequent prefetch count, the PMC 120 has done some work as described below.

Upon receipt of the VA 103, the PMC 120 performs a look-up in TAGRAM 121 to determine whether the first VA (provided by the CPU core 102) is a hit or miss in L1P and also performs a VA to PA conversion using the address translator 122. The PMC 120 also calculates a second VA (the next contiguous VA following the VA provided by the CPU core) before receiving the prefetch count 104. The PMC 120 speculatively accesses the TAGRAM 121 and uses the address translator 122 to determine the hit/miss status of the second VA, and populates register 123 with the hit/miss indication 124 and the PA 125. The valid bit 126 in the register 123 is set to a valid state to thereby permit further processing of the second VA as described above (e.g., retrieve the corresponding cache line from the L1P 130 if present, or from the L2 memory cache 155 if needed).

However, before any further processing of the second VA occurs, it is possible that the CPU core 102 sends a prefetch count of 0 to the PMC 120 meaning that the CPU core does not need any prefetch units besides the prefetch unit starting at the original VA 103. At this point, the PMC 120 is provided a prefetch count of 0, and thus the prefetch unit associated with the second VA is not needed. However, the PMC also already determined the hit/miss status of the second VA and has generated the corresponding PA. Both the hit/miss indicator 124 and the PA 125 have been stored in the register 123 by the time the zero prefetch count has been received by the PMC 120. The PMC 120 changes the status of the valid bit 126 to indicate an invalid state to thereby preclude any further processing of the second VA. This condition (valid bit set to an invalid state) is referred to as a “kill”, that is, the PMC 120 kills the processing of the second VA.

In some situations, however, the CPU core 102 may determine, despite the previous kill, that the prefetch unit associated with the second VA should indeed be obtained from the L1P 130 or L2 memory cache 155 as described above. For example, if the CPU core 102 has no further internal prediction information to inform the next required instruction address, the CPU core 102 will signal to the PMC 120 that it should continue prefetching linearly starting from the last requested address. This condition may occur, for example, due to a misprediction in the branch prediction logic in the CPU core 102. The CPU core 102 thus issues a revive signal 106 to the PMC 120 for this purpose. The PMC 120 responds to the revive signal by changing the valid bit 126 back to the valid state to thereby permit the continued processing of the second VA through the memory subsystem pipeline as described above. As such, the CPU 102 need not submit the second VA directly to the PMC 120. Instead, the PMC 120 retains the second VA in, for example, register 123 as well as its hit/miss indicator 124 thereby avoiding the power consumption and time spent to again determine the hit/miss status of the second VA and translate the second VA to a PA.

FIG. 4 shows an example of a flow chart 400 for initiating, then killing, and then reviving a memory address look-up. The operations can be performed in the order shown, or in a different order. Further, the operations can be performed sequentially or two or more of the operations can be performed concurrently.

At 402, the method includes receiving by the memory controller subsystem 101 an access request at a first VA. In one implementation, this operation is performed by the CPU core 102 providing the first VA to the PMC 120. At 404, the method includes determining if the first VA is a hit or a miss in the L1P 30. In one example, this operation is performed by accessing the PMC's TAGRAM 121 to determine the hit/miss condition of the first VA. The first VA is translated to a first PA at 406 by using, for example, the address translator 122.

At 408, the method includes computing a second VA based on the first VA. The second VA may be computed by incrementing the first VA by a value to generate an address of a byte that is 64 bytes following the byte associated with the first VA. At 410, the method includes determining if the second VA is a hit or a miss in the L1P 30. In one example, this operation is performed by accessing the PMC's TAGRAM 121 to determine the hit/miss condition of the second VA. The second VA is translated to a second PA at 412 by using the address translator 122 as described above. At 414, the method includes updating a register (e.g., register 123) with the hit/miss indicator 124 and the second PA. Further, the valid bit 126 is configured to be a valid state.

The PMC 120 then receives a prefetch count at 416. Then, at 418, if the prefetch count is greater than zero, then at 420, the program instructions from the L1P 130 or the L2 memory cache 155 (or additional level(s)) are retrieved as described above. However, if the prefetch count is zero, then at 422, the valid bit 126 is changed to the invalid state. Despite having provided a prefetch count of zero to the PMC 120, the CPU core 102 may then provide a revival indication to the PMC 120 (424). At 426, the PMC 120 changes the valid bit 126 back to the valid state and memory controller subsystem 101 then obtains the program instructions associated with the second PA from the L1P, L2 memory caches, etc. as appropriate (428).

FIG. 5 shows an example use of the processor 100 described herein. In this example, the processor 100 is part of a system-on-chip (SoC) 500 that includes the processor 100 and one or more peripheral ports or devices. In this example, the peripherals include a universal asynchronous receiver transmitter (UART) 502, a universal serial bus (USB) port 504, and an Ethernet controller 506. The SoC 500 can perform any of a variety of functions as implemented by, for example, the program instructions executed by the processor 100. More than one processor 100 may be provided and, within a given processor 100, more than one CPU core 102 may be included.

In this description, the term “couple” or “couples” means either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The recitation “based on” means “based at least in part on.” Therefore, if X is based on Y, X may be a function of Y and any number of other factors.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims. 

What is claimed is:
 1. An apparatus, comprising: a central processing unit (CPU) core; a first memory cache to store instructions for execution by the CPU core, the first memory cache having a first line size; a second memory cache to store instructions for execution by the CPU core, the second memory cache having a second line size, the second line size being larger than the first line size, each line of the second memory cache comprising an upper half and a lower half; and a memory controller subsystem coupled to the CPU core and the first and second memory caches, the memory controller subsystem being configured to: upon a determination of a first miss in the first memory cache for a first target address: determine that the first target address maps to the lower half of a first line in the second memory cache; retrieve the entire first line from the second memory cache; and return the entire first line from the second memory cache to the first memory cache; and upon a determination of a second miss in the first memory cache for a second target address: determine that the second target address maps to the upper half of a second line in the second memory cache; and return to the first memory cache the upper half of the second line from the second memory cache and not the lower half of the second line from the second memory cache.
 2. The apparatus of claim 1, wherein the second line size is twice the first line size.
 3. The apparatus of claim 1, wherein the memory controller subsystem includes: a first memory controller to determine that the first target address maps to the lower half of the first line in the second memory cache and to generate a request for the entire first line from the second memory cache; and a second memory controller to receive the request and to access the second memory cache to retrieve the entire first line.
 4. The apparatus of claim 3, wherein the first target address is a virtual address and the request for the entire first line from the second memory cache includes a physical address generated based on the virtual address and the request for the entire first line also includes an indicator that the entire first line from the second memory cache is to be retrieved from the second memory cache.
 5. The apparatus of claim 1, wherein the first target address is to be provided to the memory controller subsystem by the CPU core to retrieve a prefetch unit that includes a set of program instructions starting at the first target address, and the CPU core also asserts a signal to the memory controller subsystem that program instructions within an additional prefetch unit are not to be retrieved and provided back to the CPU core.
 6. The apparatus of claim 1, wherein a third target address is to be provided to the memory controller subsystem by the CPU core to the memory controller subsystem to retrieve a first prefetch unit that includes set of program instructions starting at the third target address and wherein the CPU core also is to provide a first prefetch count to the memory controller subsystem, the prefetch count indicative of a number of prefetch units of program instructions following the first prefetch unit.
 7. The apparatus of claim 6, wherein the memory controller subsystem is to calculate a first series of target addresses based on the first prefetch count and the second target address and wherein the first target address is the last target address in the first series.
 8. The apparatus of claim 1, wherein the determination that the first target address maps to the lower half of the first line in the second memory cache includes a determination of a logic state of at least one bit in the first target address.
 9. The apparatus of claim 7, wherein memory controller is further configured to: return to the first cache memory the entire first line in response to the first target address being the last target address in the first series defined by the first prefetch unit; and return to the first memory cache the upper half of the second line in response to the second target address being a last target address in a second series defined by a second prefetch unit and a second prefetch count.
 10. The apparatus of claim 7, wherein: the first target address and the third target address are the same; the first prefetch count is zero; and the memory controller is further configured to mark the upper half of the first line as invalid in response to the first prefetch count being zero.
 11. The apparatus of claim 10, wherein the memory controller is further configured to change the upper half of the first line from invalid to valid in response to a revive instruction.
 12. A system, comprising: an input/output device; and a processor coupled to the input/output device and including a central processing unit (CPU) core, a first memory cache, a second memory cache, and a memory controller subsystem, wherein: the first memory cache is to store instructions for execution by the CPU core, the first memory cache having a first line size; the second memory cache is to store instructions for execution by the CPU core, the second memory cache having a second line size, the second line size being lamer than the first line size, each line of the second memory cache comprising an upper half and a lower half; the memory controller subsystem is coupled to the CPU core and to the first and second memory caches; and the memory controller subsystem is configured to: upon a first miss in the first memory cache of a first target address: determine that the first target address maps to the lower half of a first line in the second memory cache; retrieve the entire first line from the second memory cache; and return the entire first line from the second memory cache to the first memory cache; and upon a second miss in the first memory cache of a second target address: determine that the second target address maps to the upper half of a second line in the second memory cache; and return to the first memory cache only the upper half of the second line from the second memory cache.
 13. The system of claim 12, wherein the second line size is twice the first line size.
 14. The system of claim 12, wherein the memory controller subsystem includes: a first memory controller to determine that the first target address maps to the lower half of the first line in the second memory cache and to generate a request for the entire first line from the second memory cache; and a second memory controller to receive the request and to access the second memory cache to retrieve the entire first line.
 15. The system of claim 12, wherein the first target address is to be provided to the memory controller subsystem by the CPU core to retrieve a prefetch unit that includes a set of program instructions starting at the first target address, and the CPU core also asserts a signal to the memory controller subsystem that program instructions within an additional prefetch unit are not to be retrieved and provided back to the CPU core.
 16. The system of claim 12, wherein a third target address is to be provided to the memory controller subsystem by the CPU core to the memory controller subsystem to retrieve a first prefetch unit that includes a set of program instructions starting at the third target address and wherein the CPU core also is to provide a prefetch count to the memory controller subsystem, the prefetch count indicative of a number of prefetch units of program instructions following the first prefetch unit.
 17. The system of claim 16, wherein the memory controller subsystem is to calculate a series of target addresses based on the prefetch count and the third target address and wherein the first target address is the last target address in the series.
 18. An apparatus, comprising: a central processing unit (CPU) core; an L1 program cache to store instructions for execution by the CPU core, the L1 program cache having a first line size; an L2 memory cache to store data and executable instructions, the L2 memory cache having a second line size that is twice the size of the first line size, each line of the L2 memory cache comprising an upper half and a lower half; and a memory controller subsystem coupled to the CPU core, the L1 program cache, and the L2 memory cache, the memory controller subsystem to: receive a first address from the CPU core, the first address corresponding to a prefetch unit that includes a set of instructions to be executed by the CPU core; receive a prefetch count from the CPU core, the prefetch count indicating a number of additional prefetch units of instructions; responsive to the prefetch count being 0 and the first address determined to be a first miss in the L1 program cache: determine that the first address maps to the lower half of a first line in the L2 memory cache; and store the entire first line from the L2 memory cache into the L1 program cache; and responsive to the prefetch count being greater than 0: calculate a series of addresses based on the first address and the prefetch count, the series of addresses including an initial address and a last address; determine that the last address in the series is a second miss in the L1 program cache; determine that the last address maps to the lower half of a second line in the L2 memory cache; and responsive to the determinations that the last address in the series is the second miss in the L1 program cache and that the last address maps to the lower half of the second line in the L2 memory cache, store the entire second line from the L2 memory cache into the L1 program cache.
 19. The apparatus of claim 18, wherein the memory controller subsystem is to: receive a second address from the CPU core, the second address corresponding to a second prefetch unit; receive a second prefetch count from the CPU core, the second prefetch count indicating a number of additional prefetch units of instructions; and responsive to the second prefetch count being 0, the second address being a third miss in the L1 program cache and that the second address maps to the upper half of a third line in the L2 memory cache, store the upper half, and not the lower half, of the third line from the L2 memory cache into the L1 program cache.
 20. The apparatus of claim 18, wherein the memory controller subsystem includes: a first memory controller to determine that the first address maps to the lower half of the first line in the L2 memory cache and to generate a request for the entire first line from the L2 memory cache; a second memory controller to receive the request from the first memory controller and to access the L2 memory cache to retrieve the entire first line; and wherein the first address is a virtual address and the request for the entire first line from the L2 memory cache includes a physical address generated based on the virtual address and the request for the entire first line also includes an indicator that the entire first line from the L2 memory cache is to be retrieved from the L2 memory cache. 